1. Field of the Invention
The present invention relates to a phase locked loop (PLL), more particularly, to a PLL with a low-variation and stable loop bandwidth.
2. Description of the Related Art
Phase locked loop (PLL) plays important roles in the communication engineering area, and its application range is quite extensive, for example, modulation, de-modulation, frequency multiplication, frequency synthesizing, carrier synchronization, bit synchronization, etc. all apply the concept and technical of PLL therein. PLL is a kind of feedback loop. In the loop, the frequency and phase of the output clock signal are locked on the same frequency and phase of the input reference clock signal by using the feedback signal. Therefore, in the radio communications, if the phenomenon of carrier frequency offset is occurred by signal transmitting, the local oscillation frequency of the receiver follows the frequency offset to achieve the purpose of phase locking by using PLL in the receiver.
FIG. 1 is a diagram of a conventional PLL 100. Referring to FIG. 1, in general, the PLL 100 includes a phase detector (PD) 101, a charge pump (CP) 103, a loop filter (LF) 105, and a voltage controlled oscillator (VCO) 107. The PD 101 would compare the phase of the input reference clock signal Ref_Clk with the phase of the output clock signal Out_Clk returned from the VCO 107, so as to generate a control signal CS to the CP 103.
The CP 103 generates an output voltage Vout according to the control signal CS. The LF 105 is composed by the resistor Rf and the capacitor Cf, and used for filtering high frequency noises on the output voltage Vout of the CP 103. The VCO 107 generates the output clock signal Out_Clk in response to the output voltage Vout, which is filtered by the LF 105, of the CP 103.
For some PLL's applications, the loop bandwidth of the PLL is requested for having very low variation, and the loop transfer function of the conventional PLL is generally determined by the gain (Kpd) of the PD 101, the current (Icp) of the CP 103, the gain (Kvco) of the VCO 107, the resistor Rf, and the capacitor Cf.
In practical, since the PD 101 may be composed by a pure digital circuit, so that the range of variation of the gain (Kpd) of the PD 101 is very low. Accordingly, the variation of the gain (Kpd) of the PD 101 has a little effect on the loop bandwidth of the PLL 100. In addition, the capacitor Cf also has a little effect on the loop bandwidth of the PLL 100, and if the current (Icp) of the CP 103 is the inverse of the resistor Rf, the current (Icp) of the CP 103 also has a little effect on the loop bandwidth of the PLL 100. Furthermore, the gain (Kvco) of the VCO 107 has a biggest effect and a hardest control on the loop bandwidth of the PLL 100. The gain (Kvco) of the VCO 107 is defined as below:
  Kvco  =            Δ      ⁢                          ⁢      f              Δ      ⁢                          ⁢      v      
Wherein, Δf is the differential of the frequency of the output clock signal of the VCO 107 in a unit time; and ΔV is the differential of the output voltage of the CP 103 in a unit time.
In general, in order to restrain the variation of the gain (Kvco) of the VCO 107 so as to no influence the variation of the loop bandwidth of the PLL 100. The structure of inductor-capacitor (LC) may be mostly used to make the VCO (i.e. LC VCO), but since the LC VCO has an up limit to restrain the variation of the gain (Kvco) of the VCO 107, and needs more layout area and higher fabrication cost, so that the die designer would not like to design such VCO into the die.
In addition, if by using low cost process, the structure of ring oscillator circuit may be usually used to make the VCO (ring oscillator VCO), but since the variation of the gain (Kvco) of the ring oscillator VCO is quite high when the temperature, voltage, and process are changed. Accordingly, the loop bandwidth of the PLL 100 also has a high variation and unstable.
Nothing herein should be construed as an admission of knowledge in the prior art of any portion of the present invention. Furthermore, citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention, or that any reference fauns a part of the common general knowledge in the art.